1. Field of the Invention
The invention relates to electric light strings.
2. Description of Related Art
Light strings are widely used for decorative purposes, especially for holiday lighting. Light strings refer to incandescent filament lamps or LED lamps connected electrically in a series. Generally, a light string includes more than 10 incandescent filament lamps, and is powered by mains electricity, such as 110V AC mains electricity. Thus, a voltage supplied on each incandescent filament lamp is 1.5V to 12V. When the light string is powered on, a surge current may be generated and may burn out the incandescent filament lamp bulb mounted in a lamp base. In the conventional light strings, when the filament of the incandescent filament lamp bulb fails and the bulb remains in the string, or when the bulb is removed from its socket for replacement, the closed path for the flow of electrical current is interrupted and the remainder of the lamps in the string will no longer be illuminated.
U.S. Pat. No. 8,324,820 for capacitor shunted LED light string discloses a capacitor shunt across terminals of each LED light. When one LED light fails, the capacitor with which the failed LED light is connected to in parallel is broken down and shorts out, thus continues current in the light string, and keeps the light string illuminated. However, the capacitor shunt has several disadvantages.
Firstly, when one LED light fails, the corresponding capacitor shorts out, and a voltage drop on the failed part (composed of the failed LED light and the shorted-out capacitor) become lower than before, thus a voltage drop cross the other LED light rises accordingly. When more and more LED lights fails, the voltage drop cross each normal LED light rises significantly. As we know, when voltage drop across an LED is twice than the rated voltage of an LED, the LED will burn up in an hour because of over-heat. Thus, when more and more LED lights fails, the remained LED lights may all burned up.
Secondly, the low break-down voltage chip capacitor is not suitable for high-power incandescent filament lamps, because the line current is much larger than that in a LED light, and the broken-down capacitor will be over-heat rapidly, and will be destroyed and invalid.
Thirdly, the manufacturing cost of the break-down capacitor is high and too pricey to be practical.